1. Field of the Invention
The present invention relates to shaping glass sheets into complicated shapes incorporating two sharp lines of bending that extend completely across a dimension of the bent glass sheet and a sagged portion intermediate said pair of sharply bent areas. Such complicated bends are necessary in the formation of glass press polishing molds used for press polishing sheets of plastic such as polycarbonate and/or acrylic resin plastics that are components of lightweight aircraft transparences and which require very smooth surfaces in order to provide optical properties that do not tire the eyes of a pilot of aircraft in which the plastic transparency is installed. It is also necessary that the glass sheets conform exactly to the shape required for the plastic transparencies to fit within the frame in which the ultimate aircraft transparency is mounted.
Glass sheets shaped by the method recited herein are shaped to provide conforming surfaces that simultaneously press polish the opposite surfaces of the plastic to be press polished as part of the fabrication process for the plastic transparency. It is very difficult to shape a glass sheet by a gravity sag bending technique without causing the sheet to sag without control, particularly in the central portion which is unsupported by an outline ring mold. When a glass sheet is sagged to conform throughout its extent to a continuous shaping surface provided by a continuous mold, the glass on contacting the mold shaping surface while heat-softened develops all of the defects that result from contacting dust particles or any irregularties on the mold shaping surface. In an effort to shape glass sheets to an exact shape throughout their extent, glass sheets have been press bent using pressing molds that engage one or both of the opposite major glass sheet surfaces to provide an exact shaping surface to which the glass sheet is to be conformed. The art has developed covers or protective blankets of fiber glass or other fibrous material, which, unfortunately, have a pattern which is likely to be imprinted on the surface of the heat-softened glass when pressurized contact is made in the vision area. The necessity for a closely conforming shape throughout the entire extent of the glass sheet to the desired shape without surface marking has brought about a need to effectively control the amount of shape imparted to a glass sheet bent to a complex shape while supported on its periphery only on an outline mold.
In the past, electric heating elements have been used in conjunction with overall heat within an enclosed hot atmosphere to provide a sharp bend between adjacent areas that are essentially flat. It has also been known to avoid too much disparity in heat between the top and bottom surfaces in order to avoiding bowing the glass sheet convexly with respect to the remainder of the sheet while using top heat.
All of the above problems have made it very difficult to shape glass sheets to complicated shapes involving a pair of sharply bent areas extending across the width of the shaped sheet and a controlled sag of essentially spherical configuration in the glass sheet region intermediate the sharply bent areas.
2. Description of the Prior Art
U.S. Pat. No. 3,293,020 to Sleighter discloses a method of heating glass sheets supported on a mold and heated simultaneously from above and below by means of heating elements located within a tunnel-like lehr for heating the supported glass sheets into the shape of the mold.
U.S. Pat. No. 2,967,378 to Jones and Black discloses a method and apparatus for bending glass sheets to compound shapes by subjecting them to different heat patterns, first of the general heat type to heat the entire surface to a uniform temperature, followed by treatment with localized intense heat. Both heat treatments are applied from above.
U.S. Pat. No. 2,377,849 to Binkert and Jendrisak discloses a process for bending glass sheets using a multiple step and multiple mold arrangement. Initially, the glass sheets are sagged by gravity into a mold of concave elevation and the bend is completed by applying suction to complete the bend. This patent also discloses shaping glass sheets to compound curvature by providing initial bending by gravity sagging over a ring mold to form a regular spherical bend, followed by removing the prebent sheets from the mold, cutting sections of the desired outline from the sheets and then placing the prebent cut sheets upon a second mold provided with a concave bending surface conforming to the final shape to be given the glass sheets. The second bend is assisted by suction.
U.S. Pat. No. 2,999,338 to Richardson discloses an outline mold for shaping glass sheets by gravity sagging to a compound bend wherein a heating element 30 is pivotally mounted over a central region of sagging and a pair of transversely extending heaters 24 located beneath transverse elongated areas of sharp bending. In addition, this patent discloses a sag bending indicator. The pivotally mounted central heater would be difficult to pivot downward if the end portions of the glass were bent to extend approximately normal to the central portion thereof. Furthermore, the central portion of the glass sags away from an overhead heater, thereby reducing the effectiveness of the central heater associated with the mold.
U.S. Pat. Nos. 682,571 to Sage; 3,560,182 to Golightly; 3,560,183 to Stilley et al and 3,607,186 to Bognar disclose the use of sloped supports disposed around the periphery of a mold that provides sliding support for the margin of a glass sheet that sags to conform to a mold shaping surface.
U.S. Pat. No. 3,069,877 and others to Golightly disclose an outline, sectionalized mold having a sloped enlongated shaping rail that supports a longitudinal side edge of a glass sheet being bent into a bend comprising a pair of sharply bent areas extending across the glass sheet and a longitudinally extending elongated area that forms a longitudinally extending side edge portion bent relative to the remainder of a glass sheet about an axis of bending that extends longitudinally of the sheet.
U.S. Pat. No. 3,356,480 to Golightly discloses the formation of a compound bend which includes two lines of bending across the transverse dimension of a glass sheet and a side portion bent about an axis extending longitudinally of the glass sheet and to one side of the longitudinal center line thereof. The glass is bent on a sectionalized mold and while still supported on the mold, is irradiated from above to provide the transverse bend about the axis extending longitudinally of the glass sheets. This patent also shows an embodiment comprising an elongated slide sloped transversely of its length to one side only of the outline old shaping surface.
U.S. Pat. No. 4,072,493 to Imler discloses an outline bending mold for shaping one or more glass sheets by gravity sag bending including one or more sharp bends by combining overall heating and localized heating using one or more electroconductive heating ribbons and guiding means to locate the ribbons below the shaping surface of a sectionalized mold in positions which would not interfere with loading or unloading glass sheets on the mold. The shapes developed by Imler are sharp bends between relatively flat glass sheet portions.
Other patents cited during novelty searches of the subject matter of this invention include U.S. Pat. No. 2,897,632 to Fowler and Dunipace which shows a sectionalized, outline mold, U.S. Pat. No. 3,114,571 to Carson and Leflet which shows a cap windshield, U.S. Pat. No. 3,264,082 to Golightly which shows a sectionalized, outline mold, U.S. Pat. No. 3,333,935 to Valchar and Mrozinski which shows local heating to effect a sharp bend in press bent glass, U.S. Pat. No. 3,348,935 to Carson and Leflet which shows various sectionalized, outline molds, U.S. Pat. No. 3,632,326 to Ritter which relates to warping glass by differential cooling followed by inertia sag bending and U.S. Pat. No. 3,918,948 to Monzi which discloses means to maintain glass sheets in alignment.
In addition, the following patents were cited as of interest in the disclosure of the formation of sharp lines of bending. These include U.S. Pat. No. 3,340,037 to Stevenson, which discloses pivoted blast guides which are used in conjunction with burners for providing oblique lines of sharp bending at an angle to the direction of glass sheet movement through a bending lehr, U.S. Pat. No. 4,043,785 to Reese, which discloses a manner of forming a sharp line of bending using an electroconductive ribbon, U.S. Pat. No. 4,047,916 to Reese, Mortimer, Tobin and Jursa, which shows the employment of weighted ribbons for forming a double V-bend on glass sheets supported on outline molds, U.S. Pat. No. 4,074,996 to Hagedorn, Rahrig and Revells, which discloses apparatus for forming sharp bends that includes a sectionalized primary press bending member having articulated end sections for sharply bending portions of the glass sheet about lines superheated by a radiant energy heat source located above the primary press member and further provided with clamping devices and a supplementary press member with a complemental shaping surface for pressing the glass end portions sharply relative to the main portion of the glass sheet, U.S. Pat. No. 4,077,791 to Oelke, which discloses a mold for forming relatively sharp angled bends and particularly discloses a special contacting member to insure good electrical contact between a source of electrical power and the heating element used to provide the intense heat to improve the sharpness of bend, and U.S. Pat. No. 4,119,425 to Marriott, which discloses a window formed of a glass sheet having a combined electric heating circuit and bending circuit imprinted thereon. The bending circuit includes resistance elements in the form of an electrically conducting path extending transversely of the sheet along the lines about which a sheet is to be bent sharply.
The glass sheet shapes developed by the prior art either required elongated areas of bend between relatively flat portions shaped about sharp axes of bending relative to one another or, in the case of precision formed compound bends, involved the sagging of the glass sheet onto an upward facing shaping surface of continuous or substantially continuous contour conforming to the ultimate shape desired. The prior art has failed to develop a technique that would avoid the harm to the bottom surface of a glass sheet bent to conform to a supporting surface of the proper shape in both longitudinal and transverse directions in the central portion of a glass sheet between transversely extending areas bent to comparatively sharp bends.
The prior art required a technique to enable one to bend a glass sheet into a shape comprising a pair of elongated areas of sharp bending extending across the transverse dimension of the sheet combined with an area of controlled spherical sagging in the central portion of the sheet intermediate the transversely extending areas of sharp bending.